Spinocerebellar ataxias (SCAs) are a group of more than 40 genetically heterogeneous neurodegenerative disorders, numerous of which are caused by CAG repeat expansions in coding regions that generate toxic polyglutamine (polyQ) tracts. Although these CAG repeat expansions drive progressive neurodegeneration, particularly in the cerebellum, the molecular mechanisms linking the mutation to disease pathogenesis remain poorly understood. Recently, dysregulation of alternative splicing has emerged as a transcriptomic hallmark of CAG expansion SCAs, yet most work to date has focused exclusively on the skipped exon (SE) event class and largely on SCA1 models. To address this gap, I performed the first comprehensive investigation of all five major alternative splicing event classes skipped exon, (SE), retained intron (RI), mutually exclusive exons (MXE), alternative 5′ splice site (A5’SS), and alternative 3′ splice site (A3’SS), across multiple CAG expansion SCA mouse models. This analysis revealed widespread missplicing across all splicing types and demonstrated that alternative splicing dysregulation is a shared transcriptomic feature of SCAs. Splicing alterations were enriched in pathways relevant to SCA pathogenesis, including neuronal structure and function, cytoskeletal processes, and ion channel regulation. Importantly, dysregulated events were highly expressed in cerebellar Purkinje neurons, supporting a direct link between alternative splicing defects and selective neuronal degeneration. To further evaluate the contribution of alternative splicing to disease progression and therapeutic response, I characterized the transcriptome of SCA3 Q84 mice in the v cerebellum and brainstem following treatment with an antisense oligonucleotide (ASO) targeting ATXN3. SCA3 mice exhibited robust dysregulation of alternative splicing and differential gene expression in both brain regions, with SE events representing the most abundant splicing alteration. ASO-5 treatment partially restored normal splicing patterns, demonstrating that alternative splicing represents a molecular marker of therapeutic target engagement. Collectively, this work establishes that dysregulation of multiple classes of alternative splicing is a transcriptomic feature of CAG expansion SCAs, broadening the current mechanistic understanding of disease pathogenesis and highlights the potential of alternative splicing as a promising biomarker.
Brianna S. Nelthrope (Wed,) studied this question.